linux-kernel - Re: [PATCH v2] uaccess: add noop untagged

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Message-ID: <20190604123759.GA6610@arrakis.emea.arm.com>
Date:   Tue, 4 Jun 2019 13:38:00 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v2] uaccess: add noop untagged_addr definition

On Tue, Jun 04, 2019 at 09:28:41AM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 04, 2019 at 02:04:47PM +0200, Andrey Konovalov wrote:
> > Architectures that support memory tagging have a need to perform untagging
> > (stripping the tag) in various parts of the kernel. This patch adds an
> > untagged_addr() macro, which is defined as noop for architectures that do
> > not support memory tagging. The oncoming patch series will define it at
> > least for sparc64 and arm64.
> > 
> > Acked-by: Catalin Marinas <catalin.marinas@....com>
> > Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> >  include/linux/mm.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0e8834ac32b7..dd0b5f4e1e45 100644
> > +++ b/include/linux/mm.h
> > @@ -99,6 +99,17 @@ extern int mmap_rnd_compat_bits __read_mostly;
> >  #include <asm/pgtable.h>
> >  #include <asm/processor.h>
> >  
> > +/*
> > + * Architectures that support memory tagging (assigning tags to memory regions,
> > + * embedding these tags into addresses that point to these memory regions, and
> > + * checking that the memory and the pointer tags match on memory accesses)
> > + * redefine this macro to strip tags from pointers.
> > + * It's defined as noop for arcitectures that don't support memory tagging.
> > + */
> > +#ifndef untagged_addr
> > +#define untagged_addr(addr) (addr)
> 
> Can you please make this a static inline instead of this macro? Then
> we can actually know what the input/output types are supposed to be.
> 
> Is it
> 
> static inline unsigned long untagged_addr(void __user *ptr) {return ptr;}
> 
> ?
> 
> Which would sort of make sense to me.

This macro is used mostly on unsigned long since for __user ptr we can
deference them in the kernel even if tagged. So if we are to use types
here, I'd rather have:

static inline unsigned long untagged_addr(unsigned long addr);

In addition I'd like to avoid the explicit casting to (unsigned long)
and use some userptr_to_ulong() or something. We are investigating in
parallel on how to leverage static checking (sparse, smatch) for better
tracking these conversions.

-- 
Catalin